1. Field of the Invention
The present invention relates to an apparatus for supplying raw material which is to be used in growing a single crystal by the Czochralski method (referred to as CZ method hereinafter), and to a method of supplying raw material which is to be employed in said apparatus. More particularly, it relates to an apparatus for supplying raw material which is to be used for additional charging or recharging of solid raw material to the molten raw material in a crucible in growing a silicon single crystal, and to a method of supplying raw material which is to be employed in said apparatus.
2. Description of the Related Art
Generally, in growing silicon single crystals by the CZ method, the solid polycrystalline silicon charged into a crucible as the initial charge is heated and melted by a heater surrounding the crucible. After formation of molten raw material in the crucible, a seed crystal held above the crucible is lowered until it is soaked into the molten raw material in the crucible, while the crucible is rotated in a fixed direction. The seed crystal is then pulled while it is rotated in a given direction, whereby a cylindrical silicon single crystal is thus grown underneath the seed crystal.
The solid raw material to be charged into the crucible as the initial charge is polycrystalline silicon in various forms, such as rod-like, lump-like or a granular form, which is supplied alone or in combination with each other, and serves to constitute the molten raw material for silicon single crystal growing.
In silicon single crystal growing by foregoing CZ method, when the solid raw material initially charged into the crucible melts down, the volume decreases, resulting in deficiency of molten raw material relative to the crucible volume capacity. If single crystal growing is carried out in such a condition, a decrease in productivity is unavoidable due to the insufficient quantity of the molten raw material.
For avoiding the decrease in productivity due to the above, it becomes necessary to secure a desired amount of the molten raw material by replenishing the deficiency of molten raw material. Thus, “additional charging” is carried out as a technique for additionally supplying the solid material after initial charging into the crucible.
Namely, this “additional charging” is a technique for increasing the amount of the molten raw material in the crucible by further adding the solid material to the molten raw material after melting the solid material initially charged in the crucible. By applying this “additional charging”, it becomes possible to efficiently make most of the crucible volume capacity to be used and thereby improve the productivity in silicon single crystal growing.
Further, in growing silicon single crystals by the CZ method, a solid raw material supplying technique called “recharging” is also employed. More specifically, this is a technique adding the solid material in an amount corresponding to the decrease of the molten raw material incurred by pulling a first single crystal, into the molten raw material remaining in the crucible after pulling and growing said single crystal.
In other words, the technique is perceived that, applying “recharging”, the quantity of molten raw material in the crucible can be maintained to the predetermined level, which makes it possible to repeatedly pull a single crystal, thus enabling the increase of the number of crystals to be pulled per crucible.
Therefore, by employing “recharging”, it becomes possible to attain cost reduction through efficient utilization of the crucible and to improve the productivity and thereby reduce the cost of growing silicon single crystals, similarly to foregoing “additional charging”.
However, the material supply by additional charging or recharging is made in the art by adding the solid material in granular form to the molten raw material in the crucible using a raw material supply device, which is inserted in the pulling/growing furnace. Therefore, it becomes more of an issue that the solid material additionally charged may damage the crucible or cause splashing of the molten raw material, the splash of which may adhere to the machinery parts in the crucible, resulting in shortening the life thereof or adversely affect the process of single crystal growing.
In this regard, various proposals concerning additional charging or recharging have been made in the art. For example, in view of the fact that the quartz crucible is readily damaged on the occasion of recharging and this may readily cause dislocation during single crystal growing, Japanese Patent Application Publication No. 09-208368 proposes a method of supplying silicon materials by which silicon materials can be additionally supplied in a manner friendly to both the silicon melt surface and solidified surface, rapidly and without damaging the quartz crucible on the occasion of melting.
It is alleged that when the method proposed in the above-cited Publication No. 09-208368 is adopted, the splashing of the molten raw material can be avoided and the productivity in silicon single crystal growing and the production yield can be improved.
Furthermore, since uncharacterized simple supply of the solid material directly to the material in the crucible causes splashing of the melt, a technology is employed which comprises the steps of solidifying the melt surface to some extent after pulling a single crystal as intended, feeding subsequently the solid raw material onto the solidified surface, and resuming melting said solidified surface as well as said solid raw material. In this case, it is necessary for the operator to visually monitor the state of solidification of the melt surface. During such monitoring, the operator cannot conduct any other work, which will be a hindrance to productivity improvement.
Therefore, for alleviating such operator's constraints and making it possible to strive for improvement in productivity, Japanese Patent Application Publication No. 11-236290 proposes a raw-material-additional-charging system for use in single crystal pulling apparatus, wherein the state of solidification of the melt surface is detected by means of a visual sensor. This raw-material-additional-charging system will make it possible to lighten operator's workload and allow the operator to conduct some other work and thereby improve the productivity.
Also, in republished WO2002/068732, since it may occur to damage the quartz crucible depending on the extent of solidification of the molten surface, which may result in peeling off the inner surface of the quartz crucible and then generating dislocation in the silicon single crystal, or may result in cracking of the quartz crucible and then generating the leakage of the molten raw material to the outside, a recharge pipe is proposed, wherein it can be easily put in place or retrieved from the single crystal pulling apparatus, and it enables the raw solid material to be directly supplied onto the solidified surface of the molten raw material in the crucible. By using this recharge pipe, a single crystal free from dislocation can be safely and efficiently produced.